Old Diesel Engine Tickover

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Fri 19 Sep 2003 14:42
Old Diesel Engine Tickover - doctorchris
OK, maybe a bit of a funny question, but why did old diesel engines (buses, boats etc) have a strange "surging" tickover that sounded rather like "thrum thrum thrum". Sometimes my Terrano's tickover will briefly do the same although I don't think this is a problem.
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Fri 19 Sep 2003 14:57
Old Diesel Engine Tickover - Cliff Pope
I always assumed it was because they really did 'tick-over' ie they were running at very nearly the slowest speed possible, and relying on the heavy flywheel to carry the crank on to the next firing piston. The speed would therefore have had a regular rhythym rather than a constant noise.
Other reasons might be imbalance between the injectors, perhaps needing cleaning, or play in the governor mechanism allowing the engine to 'hunt'. My petrol tractor used to do this until I adjusted the governor
The amount of diesel used at tickover was minute, hence the old habit of leaving the engines running rather than restarting.
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Fri 19 Sep 2003 16:32
Old Diesel Engine Tickover - Dizzy {P}
This idle speed variation was called 'hysteresis' and was typical of diesel engines that had throttle governing. The governor would attempt to maintain a constant idle speed but couldn't quite manage it, hence it would open the throttle a touch in order to speed up a slowing engine or close it a little to hold back an accelerating engine. If you look at one of these old diesel engines when it is at idle you can see the throttle mechanism moving as it seeks to control the idle speed.

I assume that it was easier to maintain a constant idle in old petrol engines because they varied the amount of air, as well as the fuel, that entered the cylinders. Diesel engines receive a full charge of air at all engine speeds and so rely purely on metering a miniscule charge of fuel, which was obviously quite difficult in the days of basic mechanical governing.
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Sat 20 Sep 2003 22:18
Old Diesel Engine Tickover - DL
Funny this should come up - I heard a truck ticking over yesterday outside work (fairly modern-ish one) and it was doing this!

Must have something wrong with the governor I reckon.

Think it was a Ford truck, a heating oil tanker.
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Tue 30 Sep 2003 08:41
Old Diesel Engine Tickover - Dizzy {P}
< Must have something wrong with the governor I reckon. >

I think you're right. I asked a professional diesel engineer (albeit railway diesels) about this and he said that it can happen with mechanical or hydraulic governing (where the fuel rather than the air is metered) if the governor isn't set up right.

I assume that throttle governing died out around 1960. The Perkins P4 and P6 had throttle governing and they were replaced by the 4.236 and 6.354 at around that time, these having mechanical or hydraulic governing and no throttle on the air intake.
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